International Publication WO98/12098, for example, discloses a well-known vehicle provided with a pair of right and left crawler traveling devices having respective drive axles differentially connected to each other through a differential mechanism, wherein power is inputted to a traveling HST so as to drive right and left crawler traveling devices for traveling of the vehicle, and wherein power of the steering HST is inputted to the differential mechanism so as to make a difference of rotary speed between the right and left crawler traveling devices for turning of the vehicle.
In the vehicle, the traveling HST and the steering HST are provided with respective hydraulic servomechanisms for controlling positions of respective movable swash plates. Each of the servomechanisms includes an electronically automatic control valve and a manipulated control valve. The manipulated control valve of the traveling HST is interlockingly connected through a mechanical linkage to a lever (or pedal, etc.) for speed change, and that of the steering HST to a steering handle (a steering operation tool in this document, however, it may be a lever, etc.). Such manipulation devices are manipulated so as to control the manipulated control valves, thereby controlling the positions of the movable swash plates, respectively. It can be read in the document that the automatic control valves are provided for adjusting the positions of the respective movable swash plates controlled by manipulation.
Many of such constructed vehicles use the traveling HST as a main speed change mechanism and have a multi-stage sub speed change mechanism including a plurality of speed changing gears or hydraulic clutches, which serves as a transmission system interposed between the traveling HST and the differential mechanism. In this case, the above-mentioned speed change lever for controlling output of the traveling HST serves as a main speed change lever. Additionally, a sub speed change operation device (like a lever or a switch) is provided separately from the main speed change lever so as to select a speed stage of the sub speed change mechanism. The vehicle disclosed in the cited document also has such a structure.
If output of the traveling HST and output of the steering HST are controlled individually, i.e., if the movable swash plate of the traveling HST is controlled independently of manipulation of the steering handle, or if the movable swash plate of the steering HST is controlled independently of manipulation of the speed change lever, the vehicle turns at the speed set by the speed change lever regardless of the manipulated degree of the steering handle. Therefore, the vehicle does not decelerate even if the steering handle of the vehicle traveling fast is turned to the limit. Such a turning is unstable and may be dangerous.
It is assumed that output of the steering HST is decided uniformly according to the steering angle of the handle regardless of the setting of the speed change lever. Even if the steering angle of the handle is constant, a ratio of output of the traveling HST to that of the steering HST becomes large in high-speed traveling, thereby making a turning radius large. On the contrary, in low-speed traveling, the ratio becomes small, thereby making the turning radius small. In this way, sense in steering operation and accuracy in turning of the vehicle become wrong.
The output of the steering HST is used for acceleration of a turning-outside crawler traveling device and deceleration of a turning-inside crawler traveling device, thereby turning the vehicle. The larger the steering angle becomes, the smaller the driving speed of the turning-inside crawler traveling device becomes, and it reaches zero. If the steering angle is further increased, the rotation of the turning-inside crawler traveling device is reversed. Such a turning while the turning-inside crawler traveling device is stationary or rotated reversely is called a brake turn. If the output of the steering HST is independent of the output of the traveling HST as mentioned above, the steering angle at the time of start of the brake turn, i.e., when the turning-inside crawler traveling device stops, varies according to variation of output of the traveling HST. Thus, a driver must manipulate the steering handle to adjust the angle thereof troublesomely when the brake turn is going to be done.
Then, in the vehicle disclosed in the cited document, the linkage between the speed change lever and the manipulated control valve for controlling the position of the movable swash plate of the traveling HST is mechanically interlocked with the linkage between the steering handle and the manipulated control valve for controlling the position of the movable swash plate of the steering HST, so that the traveling speed varies correspondingly to the steering angle of the handle. Basically, the larger the steering angle becomes, the smaller the output of the traveling HST is made so as to decrease the real vehicle-center speed. Furthermore, the larger the stroke of the speed change lever becomes (toward the maximum speed limit), the larger the deceleration degree becomes, so that, when the steering angle is adjacent to its maximum, the real vehicle-center speed is almost constant regardless of the setting position of the speed change lever.
However, in this interlocking structure, while the output of the traveling HST varies correspondingly to the position of the speed change lever and the steering angle of the handle, the output of the steering HST varies correspondingly to only the steering angle. That is, while the steering angle is constant, the output of the steering HST is constant whether the output of the traveling HST increases or decreases, so that the ratio of output between the steering HST and the traveling HST at any steering angle varies according to the position of the speed change lever. Thus, it is impossible to make the turning radius at every arbitrary steering angle exactly constant however the position of the speed change lever, i.e., the traveling speed varies. Further, the above-mentioned object, that is, to make the steering angle at the starting of brake turn constant, is not achieved.
It is unfit for a small crawler vehicle to further improve the cited mechanical linkage for achievement of the above objects, because the improvement further complicates the linkage, expands a space for it, and increases costs.
The cited vehicle travels at approximately constant speed while the steering angle is extremely small. However, if the steering angle exceeds a certain value, the traveling speed reduces proportionally. Thus, for example, the reduction ratio of traveling speed to the increase of steering angle, which is good when the steering angle is large to some degree, is felt too large when the steering angle is small. Namely, when the steering angle is not so large and even if it is changed a little, a great reduction of traveling speed is felt and a turning circle of the vehicle becomes too small.
In this way, mechanical connection of output of the traveling HST with output of the steering HST is necessarily inaccurate in control, or it must be further complicated for enhancing the accuracy. Such a complicated structure is not acceptable with respect to an arrangement space and costs.
Furthermore, for stopping such a vehicle having the HSTs, instead of disengagement of a clutch, the speed change lever is set to the neutral position so as to stop output of the traveling HST, the steering handle is returned to the straight traveling position so as to stop output of the steering HST, and then, a brake is actuated so as to stop the left and right axles surely. However, when the brake must be hit, neither the traveling HST nor the steering HST is put into neutral, thereby advancing wearing of the drive axles. Moreover, if the neutralization of the traveling HST and the steering HST after braking is forgotten, the outputs of both the HSTs rebound suddenly when the brake is released. However, it is troublesome to put both the speed change lever and the steering handle into the respective neutral positions every braking operation.
On the other hand, it is hard for a driver to comprehend the relationship between the position of the speed change lever and the traveling speed because the speed change lever changes the traveling speed steplessly. Therefore, it is difficult to re-create the traveling speed after the speed change lever is returned to the neutral position for stopping the vehicle.